Piezoelectric resonators are often used in frequency control applications that require precise control of the resonant frequency of the resonator. For the common thickness mode resonator, the resonant frequency is a function of the piezoelectric material properties and the thickness of the resonator plate (i.e., the resonant material between the electrodes). The material properties include the elastic stiffness and its density. However, the resonator thickness is the most critical feature in determining the resonator's resonant frequencies during fabrication. The frequency-thickness product 1600 MHz*um, where the thickness is given in microns, approximates the resonant frequency of an AT cut quartz piezoelectric resonator. Other materials have appropriate frequency thickness products.
Resonators are typically manufactured using conventional grinding, lapping, etching and deposition techniques to produce a resonator plate thickness that roughly corresponds to a desired resonant frequency. For many resonator applications, in which the preciseness of the frequency is not critical, this is sufficient. For more demanding applications, the thickness must be adjusted more accurately by a process known as "trimming." In trimming, the resonator's final thickness is controlled by making slight changes in the thickness and then checking the resonant frequency. The resonant frequency may be determined in a number of ways, such as by applying a variable frequency signal to the resonator and determining its impedance. This iterative process continues until the desired resonant frequency is obtained.
Present trimming techniques, however, have a number of drawbacks. Monitoring the resonant frequency requires sophisticated, expensive equipment. Moreover, precisely trimming the resonator in response to the monitored frequency signal is difficult. For example, producing a fundamental mode quartz resonator with a resonant frequency of 400 MHz requires trimming the resonator to a thickness of 4 um. This is extremely thin, and even a slight over-etching or over-depositing in the process would produce a resonator of the wrong thickness and wrong resonant frequency. Moreover, it is not practical to measure a quartz crystal's thickness directly to the degree of accuracy required for determining its desired resonant frequency.
An object of the invention, therefore, is to provide a fabrication technique that trims a resonator to the precise thickness for producing a desired resonant frequency. Another object of the invention is to provide a fabrication technique that requires little or no monitoring of the resonant frequency in the process of trimming the resonator to produce a desired resonant frequency.